1. Field of the Invention
The present invention relates to a television intermediate frequency filter constructed in accordance with the surface wave principle and comprising an input transducer and two output transducers constructed in interdigital form and arranged symmetrically with respect to the input transducer.
2. Description of the Prior Art
Filters designed in accordance with the surface wave principle, in which surface waves are produced with the help of so-called interdigital transducers, are well known in the art. If the teeth of the interdigital transducers have a constant overlapping range, a frequency transmission curve will result corresponding to the function y = sin f/f.
This frequency transmission curve, however, has a band width which is too narrow for practical applications. Furthermore, the selectivity is not sufficiently high. It was therefore attempted to produce transducers having a broader transmission curve and better selectivity. For this purpose, various paths were tried. Either the overlapping range of the transducer teeth was varied, as is described for example in the German Offenlegungschrift No. 2,238,845, or several transducers having differing resonant frequencies were connected in parallel, as is described for example in the German Offenlegungsschrift No. 2,018,308 which corresponds to the British Letters Patent No. 1,322,833.
The techniques described previously result in filter pass curves which are symmetrical about a center frequency. Asymmetric pass curves were produced, for example, by making the spacing of the individual transducer teeth different from one another. Asymmetric frequency paths have also been obtained in the past by way of connecting several transducers in parallel.
The capacitive cross talk between input transducers and output transducers provide difficulties in the case of these simple filters which operate on the surface wave principle. This capacitive cross talk will deteriorate, in particular, the selectivity in the stopband range. It has thus already been suggested to obtain a compensation of the capacitive portions by way of a suitable arrangement of the electrodes upon the substrate. Such an arrangement is shown, for example, in the German Offenlegungsschrift No. 2,235,233. In this type of arrangement, two output transducers are provided which are arranged symmetrically with respect to the input transducer. The "hot" electrodes of the input transducer and output transducers thereby have as great a spacing from one another as possible. In addition, screening strips are inserted in the connection line between the contact points of the "hot" electrodes, which strips are connected to ground between the transducers.
The technique for lowering the capacitance between the input transducer and the output transducer, which is suggested in itself, and which resides in separating the transducers as far as possible from one another, requires an undesirably large substrate and a great pass band attenuation, since the ultrasonic beam fans open on its path from the input transducer to the output transducer, due to unavoidable difractions. For this reason, other techniques for lowering the capacitive cross talk must be given.